What reg plate for a Bike engined car?

Certain things aren't mean't to be understood.

The DVLA is one of them.

What you said is the official position, that a substatial majority of the donor car must be used to get an "age related" plate, that is a plate from the same year as the donor car but NOT the donor's own registration. The prototype Roadster has an age related plate.

My brother-in-law's car is 100% one donor car, yet he was issued with a Q plate.

Some people manage to get an age related plate despite using different engines/ axles or whatever.

The problem is that the decision is down to the local DVLA office that deals with your registration, which means no consistency.

The other possibility with the car on ebay is that someone has gone through the SVA test and registration with the donor engine and then swaped to a bike engine, this is quite common.

Cheers

Chris